Electric waveguide construction



Nov. 27, 1962 TEA NSMITTER OF A. E. KARBOWIAK ELECTRIC WAVEGUIDE CONSTRUCTION Filed Aug. 5, 1955 ELECTROMAGNETIC WAVE ENERG'Y IN THE H01 MODE swag.

n RECEIVER Hgg-QANDS J FEET Inventor A. 5.. KARB OWIAK A ttorney two points will involve two types of bends.

rates This invention relates to a high frequency electrical waveguide construction. More particularly, it relates to a waveguide construction specially suitable for use in a transmission system wherein the energy is propagated as a wave of the circular H mode.

The invention finds application, inter alia, in long haul transmission systems in which energy is transmitted in the circular H mode between points'whose spacing is of the order of kilometres, as for example in an intercity communication scheme.

If a circular guide carrying the H mode is perfectly straight and its circular geometry is also perfect, then the propagation of the H mode is undisturbed provided the guide walls are made of a homogeneous conductor. One of the most attractive features of such a guide is its low attenuation: the larger the pipe diameter and the higher the frequency the lower the guide attenuation. Unfortunately the H mode is degenerate with the E mode, and it can be shown that bending of the guide brings about a coupling between the two modes, with a consequent loss of power; thus low attenuation, a feature of the straight guide, is lost. It is known that for moderate radii of curvature the energy carried in a curved metal guide is carried alternately in the H and E modes and the effective average attenuation of the system is then the mean of the attenuations of the H and E modes; this is many times larger than the attenuation of the H mode. For very large radii of curvature the attenuation of the guide tends, for increasing radii, towards the attenuation of the pure H mode.

A long haul waveguide run for communication between (I) the intentional bend and (II) the unintentional bend.

The first type arises when one wishes to make a deliberate and sharp change of direction of the propagation (eg to negotiate a fixed obstacle). The design of such a bend does not bring any formidable problems; even a substantial increase in attenuation in the bend is tolerable because the overall performance of the system is little afiected by it (since the bend forms but a small proportion of the total guide run). There are a number of known designs for this type of bend.

The unintentional bend arises, or may arise, anywhere along the whole of the guide run, and is unavoidable for practical reasons because it is desirable for the guide to follow the contour of the ground (e.g. when laid in a trench) and in addition there is a limit to the precision with which a guide can be laid to approximate to a straight run. To achieve the goal any modification of the guide that removes the degeneracy between the H and E modes (as in the case of the intentional bend) is a step in the right direction. However, any modification of the guide that increases at the same time the guide attenuation appreciably is of no avail (in contrast to the case for intentional bends) since over a long run the guide would be considerably degraded in its performance.

It is accordingly an object of the present invention to provide a waveguide structure which will enable wave energy to be efficiently transmitted over a long waveguide run which does not follow a strictly linear path.

In order to achieve this object there is provided according to the present invention, a high frequency electrical waveguide comprising a hollow metal conductor of circular interior cross-section, in which the inner surface of tent ' V 3 ,066,Z68 Patented Nov. I 2 1962 said conductor it so formed or treated as to substantially increase the reactive component of the surface impedance thereof without materially increasing the resistive component of said impedance relative to the values which would be obtained in the absence of said forming or treatment.

The invention will be better understood from the following general discussion, read in conjunction with FIG- URE 1 of the accompanying drawing, which figure illustrates in cross-section an embodiment of the invention.

FIGURE 2 illustrates a microwave transmitting system including waveguide 1.

A known method of mathematical analysis of propagation in a curved waveguide has been extended to apply to a waveguide of arbitrary surface impedance It follows out of the analysis that the performance of a curved waveguide depends critically on the value of Z, and in particular it can be shown that, while the attenua tion of a straight waveguide is proportional to surface resistance (R,), the increase in the attenuation due to curvature is not only proportional to R but also inversely proportional to the square of the absolute value of the surface impedance |Z It is thus evident that any guide surface modification that increases X leaving R substantially unaltered will improve the performance of a curved waveguide. On the other hand any modification of the guide surface that brings about an increase in X and a proportionate increase in R is of no avail as far as the performance of a long guide is concerned. Thus replacing a copper guide by a brass guide would reduce but the proportionate increase in would be objectionable. A randomly corrugated or a rough surface would lead to similar results. We must thus seek a means of surface loading so that the added surface impedance (Z =R -l- 'X possesses the desirable property:

Now, it can be shown (see Proc., I.E.E., vol. 101, part 111, No. 72, July 1954, page 238, Theory of Composite Guides: Stratified Guides for Surface Waves, by A. E. Karbowiak) that a metal surface coated with a thin skin of low-loss dielectric material fulfils the above condition, since the surface reactance due to coating is given by:

where k and Z are respectively the propagation constant and impedance of free space, t is the thickness and K is the relative permittivity of the dielectric coating. The ratio R /X is given by:

It will be observed from Equation 2 that for example in the 8 mm. waveband a coating of polyethylene of the order of microns thick is suflicient for most practical cases, and from Equation 3 it can be shown that the condition (l) is satisfied for all low loss dielectrics.

Referring now to FIG. 1, this illustrates in cross-section (not to scale) a waveguide constructed in accordance with the invention. The hollow metal conductor 1 of interior diameter D, encloses a gaseous transmission medium, in the present example air (conventionally symbolised on the drawing by the dot 2), through which wave energy may be propagated in the circular H mode. In order to enable the waveguide run to follow unintentional bends (of the order of a few hundreds of metres radius), there is provided at the interface between conductor 1 and medium 2 means for loading the surface impedance of the inner surface of conductor 1 such that the added impedance has a reactive component substantially greater than its resistive component.

This loading means here takes the form of a layer 3 of low loss dielectric material, the thickness t of the layer 3 being chosen according to the minimum radius of curvature which the waveguide is intended to follow. Usually the metal conductor 1 is made of copper and the dielectric layer 3 may be of polyethylene or similar low-loss material. In certain circumstances it may be preferred to use a conductor of aluminium instead of copper, in which case the dielectric layer 3 may conveniently be of alumina formed on the conductor by an anodising process as well known in the metallurgical art.

For an appreciation of the advantage obtained by the use of the present invention, reference may be made to the theoretical data set out in the attached Table for two waveguides both of internal diameter D=7 cms. and operation in the circular H mode at a frequency of 34,500 mc./s., wavelength 8.7 mm., one guide using a copper conductor loaded with a layer of polyethylene of thickness t=25 microns, the other using an aluminium conductor loaded with a layer of alumina of thickness t= 25 microns. In the table the magnitude of the bend which can be tolerated, at the expense of a 10% increase inthe attenuation constant as compared with that of the, unloaded straight conductor, is given once in terms of radius of curvature, and again as the deviation from the straight course which would be observed at the centre of a 10 lengthof guide whose ends were on said course. Taking for example the copper guide, it will be seen that the 10% tolerance is reached when the radius of curvature of the bend is reduced to 20 km., which means that the guide-run must not deviate from the straight by more than $0.6 mm.-a prohibitive condition. When the guide is loaded, however, the radius of curvature may be reduced to 380 ms. i.e. the permissible deviation from the straight becomes as much as i 3'cms. in 10 metres, a figure which allows a considerable degree of freedom in installation.

TABLE Conductor material Copper Aluminrum Internal diam. of conductorcms 7 Loading dielectric material polyalumina ethyl- 7 one Thickness of dielectric microns- 25 25 Attenuation constant for straight unloaded guide H Clbkm 0.74 0.93 Increase in attenuation constant of straight guide due to loadingnl percent 1. 4 1 Radius of curvature of unloaded guide giving increase in attenuation constant km 16 Deviation from straight course over distance of 10 m. to give 10% increase in attenuation constant for unloaded guide, .mm $0.6 5:0. 8 Radius of curvature of loaded guide giving 10% increase in attenuation constant m. 380 190 Deviation lrfom straight; course over distance of IQ m. to give 10% increase in attenuation constant for loaded guide ,cms :|':3 i6. 7

When the thickness of the dielectric coating on the inside of the guide is increased to 250 microns or more, then the guide can be made to negotiate intentional bends. Thus in the above example if a 100% increase in attenuation over an arc of the bend is tolerable then a coating of polyethylene of approximately 600 microns thickness will make bending radii as small as 5 meters permissible.

Apart from a slight increase in attenuation (in the above example about 1.4% in the case of a micron coating) the introduction of the dielectric coating has a negligible efiect on the performance of a straight guide and the important feature of the guide, its Wide band, is preserved. The coating (when thin) introduces a negligible change in the characteristic impedance of the guide. Consequently a coated pipe if desired can be followed by a plain pipe, or vice-versa, without fear of reflection loss.

It will be observed that the introduction of the dielectric coating as surface loading makes the use of large diameter pipes possible and their consequential low attenuation can be exploited; this would not be possible in the case of plain pipes on account of their poor performance in other than very straight runs.

While the principles of the invention have been described above with reference to particular embodiments, it is to be clearly understood that such description is made only by way of example and not as a limitation on the scope of the invention.

What I claim is:

1. In an electromagnetic waveguide transmission system, a source of circular H mode electromagnetic wave energy, a high frequency electrical waveguide supportive of said mode connected to said source comprising a continuous hollow metal conductor of circular interior cross-section and long relative to the wave length of said energy, and a layer of low-loss dielectric material having a thickness of between 25 and 600 microns on the inner surface of said conductor to substantially increase the reactive component of the surface impedance there.- of without materially increasing the resistive component of said impedance whereby over long distances and at imperceptible bends no substantial mode conversions are produced.

2. A high frequency electrical waveguide for trans.- mission of Wave energy having a circular H mode and constituting a uniform'transmission line for long dis tance communication in the extremely high frequency operating range and provided with loading to permit bends in said waveguide Without an undue increase in attenuation comprising a continuous hollow alum num conductor of circular interior cross-section, long relative to the wave length of said energy, and adapted to define a path in a gaseous medium for wave energy propagated in the circular H mode, a layer of low loss alumina material having a thickness of between twenty five and six hundred microns located as a continuous film on the interior surface of said waveguide for providing a substantial increase in the reactive component of' the waveguide impedance with a negligible increase in the resistive component of the waveguide impedance, to permit a reduction in the radius of the curvature of the waveguide to a value of between substantal'y and 4 of the radius of curvature of the unloaded waveguide with an increase in attenuation of between 10 percent and percent at said bend over the attenuation of the straight waveguide. 1 I

References Cited in the file of patent I UNITED STATES PATENTS 2,436,421 Cork Feb. 24, 1948 2,440,691 Jira May 4, 1948 2,695,255 Avery Nov. 23, 1954 2,724,672 Rubin g Nov. 22, 1955 2,762,981 Morgan Sept. 11, 1956 2,767,239 Kennedy Oct. 16, 1956 2,848,696 Miller Aug. 19, 1958 

